The present invention relates to molded case circuit breaker enclosures. More particularly the present invention relates to a base and mid-cover configuration for circuit breaker enclosures.
Circuit breaker enclosures are typically manufactured having a deep base for enclosing the components of a circuit breaker (i.e., a circuit breaker cassettes) and a cover having an opening for an operating handle (a bifurcated case). In existing circuit breaker enclosures, the base is constructed with sidewalls that extend to a height approaching or exceeding the height of the circuit breaker cassettes when upon the floor of the base. Other designs employ a divided cover, where in a mid-cover connects to the base and the top cover connects to the mid-cover and has an opening for the operating handle (a trifurcated case). The mid-cover may also comprise accessory recesses and/or trip actuator recesses positioned in mechanical cooperation with the operating handle of the circuit breaker. As with a bifurcated case, typical sidewalls of trifurcated case bases approach or exceed the height of the circuit breaker cassettes.
Typically, in trifurcated circuit breaker enclosures the height of the mid-cover wall is small in relation to the height of the base sidewalls. This is likely due to the limited purpose of existing mid-covers, i.e., to house accessories and trip actuators, which generally have a small height in comparison to the circuit breaker cassettes. The ratio of the mid-cover height Hmc to the base height Hb (Hmc/Hb) in prior circuit breakers molded case is less than about 0.45.
However, with relatively small Hmc/Hb ratios, manufacturing is difficult as the base must be relatively high for a very thin structure. This creates problems in the molding of the thermoplastic materials and increases the effort required to provide a finished product having thermoplastic resin and filler evenly distributed.
Further, a structural problem arises due to the high, thin sidewalls of the base. As gases are exerted when the circuit breaker cassettes trip, the stresses created cause the circuit breaker enclosure to deform and possibly crack at the base sidewalls.
Therefore, a need exist for a molded circuit breaker case that is easier to mold and that provides suitable structural integrity.
A molded circuit breaker enclosure is provided having a base, a mid-cover, and a top-cover. The base includes generally parallel sidewalls, a bottom wall, a lined end dividing portion. The mid-cover comprises generally parallel sidewalls, and a load end dividing structure. Circuit breaker components, such as one or more circuit breaker cassettes, are positioned within the base of the circuit breaker. A mid-cover is removably affixed on top of the base sidewalls, and are secured there to, for example, on a pressed fit frictional engagement, interference engagement, fastener adhesive or any combination thereof. The top-cover has a slot to allow a handle to extend therethrough.
The circuit breaker enclosures must withstand mechanical stresses caused by the high gas pressure generated when a short circuit occurs. Accordingly, it is beneficial to transfer the stresses from the sidewalls of the base to the cover or the combined mid-cover and top-cover assembly. This may be accomplished by providing a step on the upper edge of the sidewalls of the base and a corresponding step on the bottom edge of the mid-cover sidewalls in a trifurcated case or cover sidewalls in a bifurcated case.
In an exemplary embodiment of the present invention, the ratio of the midcover sidewall height to the base sidewall height is at least 0.45. In a most preferred embodiment, the ratio is between 0.61 and 1.63.